The second iteration of CSS Grid is already in the works and the public editor's draft was released last week! While it is by no means the final W3C recommendation, this draft is the start of discussions around big concepts many of us have been wanting to see since the first level was released, including subgrids:
In some cases it might be necessary for the contents of multiple grid items to align to each other. A grid container that is itself a grid item can defer the definition of its rows and columns to its parent grid container, making it a subgrid. In this case, the grid items of the subgrid participate in sizing the grid of the parent grid container, allowing the contents of both grids to align.
The currently defined characters of subgrid items are particularly interesting because they illustrate the differences between a subgrid and its parent grid. For example:
The subgrid is always stretched in both dimensions in its subgridded dimension(s): the align-self/justify-self properties on it are ignored, as are any specified width/height constraints.